1. Field of the Invention
This invention relates to pressure relief means, and in particular to such means providing fluid crossover and fluid dumping functions.
2. Description of the Prior Art
In one form of hydraulic system, a working element, such as a snowplow, is held at a desired angular position by a pair of single acting cylinders connected to the snowplow blade through the piston rods thereof. The plow blade may be pivoted about a vertical central axis with the disposition of the blade being fixed by blocking the hydraulic connections with the blade in the desired disposition.
Adjustment of the plow blade disposition is effected by causing hydraulic fluid delivery to one or the other of the cylinders through a conventional direction control valve which may comprise a spool-type valve. The blocking condition in such a spool-type valve is effected by spring biasing the spool to a centered neutral position.
A problem arises in such hydraulic control systems in that, at times, an overload may be applied to a portion of the plow blade so as to produce a high pressure condition in one or the other of the cylinders. Unless some dissipation of this high pressure condition is provided, failure of the components of the system may result from the impact surges.
It has been conventional to provide some pressure relief means to provide a crossover relief between the cylinders under such abnormal high pressure conditions. It has also been conventional to provide some high pressure relief when both cylinders reach a high pressure condition as caused by a high load being applied concurrently thereto through the blade. Conventionally, such blades not only pivot about a vertical axis at the center thereof, but also about a horizontal axis at the bottom of the blade as a result of the concurrent forces applied thereto.
Heretofore, the control of such overload conditions has required the use of three different crossover relief valves resulting in a relatively expensive and complicated structure which, because of a large number of moving parts, further presents a serious maintenance problem.